In analog integrated circuits or mixed-signal design area, the reference voltage source is a very important and commonly used module, which is widely applied to circuits, such as AD/DA converter, power converter, power amplifier, and so on. The reference voltage source has the function of providing the system with a voltage reference which is independent of the temperature and power supply. As the supply voltage decreases, the design of the reference source with low-voltage and low-power, low temperature coefficient and high PSRR becomes critical. At present, the voltage reference circuit using low voltage power supply and having low power consumption has special and important significance. With the gradual increase in popularity of mobile electronic devices, the power supply voltage of the analog integrated circuit is required to be reduced to about 1V, and power consumption is required to be at a level of uW. Therefore, the design of the reference source with low temperature coefficient, low power consumption and high PSRR is very important, and it is the future direction of development.
Like the bandgap reference, two voltages are required for producing the final output reference voltage. One is the voltage with a positive temperature coefficient, and the other is a voltage with a negative temperature coefficient. The output reference voltage with nearly zero temperature coefficient can be produced by adding these two voltages in a certain ratio. Different from the conventional bandgap reference circuit, the voltage with positive temperature coefficient changes from ΔVBE to ΔVGS, and the voltage with negative temperature coefficient is produced by the threshold voltage VTHN of NMOS transistor. As shown in FIG. 1, the block diagram of implementation of subthreshold based CMOS voltage reference circuit normally comprises 5 parts. The biasing part of the circuit is configured to provide a sub-threshold current for the circuit. The start-up circuit is configured to solve the zero-state problem of the circuit. The ΔVGS generation circuit utilizes the drain-source current characteristics of the subthreshold MOSFET to produce a voltage with positive temperature coefficient. The VCTAT generation circuit produces a voltage with negative temperature coefficient. The final reference voltage is obtained by adding the positive and negative temperature coefficient voltages mentioned above in a certain proportion.
The principle of ΔVGS generation circuit can be represented as follows:
The drain-source current of the subthreshold MOSFET can be represented in equation as follows:IDS_sub=μCOXW/L(m−1)VT2eVGS−VTH/mVT(1−e−VDS/VT)  (1)where μ is the mobility, Cox is the gate oxide capacitor per unit area, in is the reciprocal of the gate and channel surface coupling factor, VT is the thermal voltage, W and L are the width and length of MOSFET respectively, and VTH is the threshold voltage of MOSFET.
The last part of the equation can be approximated to 1 when the MOSFET drain-source voltage VDS is greater than 0.1V. Therefore, the drain-source current of the subthreshold MOSFET can be represented in an equation as follows:IDS_sub=μCOXW/L(m−1)VT2eVGS−VTH/mVT=I0W/LeVGS−VTH/mVT  (2)
The linear equation in relation to thermal voltage VT, i.e., the PTAT voltage, can be obtained through the difference between the gate-source voltages VGS of two subthreshold MOSFETs which are proportional to the drain-source currents.
The conventional subthreshold voltage reference circuit can be explained in FIG. 4. In the conventional sense, VGS of the subthreshold MOSFET that is proportional to two drain-source currents is considered as a liner positive temperature coefficient voltage, which can be represented in an equation as follows:ΔVGS=mVT ln N  (3)where N is the ratio of the drain-source currents of the two subthreshold MOSFETs, VT is the thermal voltage, and m is the reciprocal of the gate and channel surface coupling factor.
Actually, in is not independent of the temperature. m shows positive temperature characteristics at high temperature, particularly in a temperature range of 85° C. or higher. Therefore, the conventional subthreshold reference circuit mores the variation of m, resulting non-optimized temperature characteristics of the circuit. ignoring the variation of m means the conventional subthreshold reference circuit has a narrow applicable temperature range. On the other hand, the power consumption of conventional reference is on the level of μW. There is also a large space for optimization relative to the level of μ W, even to the level of pW.